This invention relates to measuring devices for two-dimensional photon-caused or corpuscular-ray-caused image signals with levels in the quantum number order which can vary with time, and especially to measuring devices for those picture signals when photons, electrons, IR, UV, X-rays, .gamma.-rays, ions, or neutrons are incident on a two-dimensional addressable area.
A television camera utilizing high-sensitivity imaging tubes has conventionally been used to detect optical images of extremely low intensity.
A silicon electron-multiplier vidicon consisting of a photoelectric layer, a silicon semiconductor target located opposite said photoelectric layer, and an electron gun used to scan said silicon semiconductor target by means of the electron beam, or another silicon electron-multiplier vidicon with an image intensifier in front of said photoelectric layer, has been used as an imaging tube in said camera.
Said television camera can be used to pick up images under illumination above a threshold on the order of several tenths of milli-luxes to several milli-luxes. Such illumination corresponds to 10.sup.8 to 10.sup.9 photons/cm.sup.2 .multidot.sec on the photoelectric layer. If a read operation is stopped while an optical image is incident on the photoelectric layer in this type of camera, signals corresponding to the optical image on the photoelectric layer can be stored in the target and it increases the sensitivity of the camera. As well as photoelectrons, the target, however, may store thermal electrons which occur in the photoelectric layer and target itself. In addition, charges on the target can leak. Low contrast causing inferior picture quality in spite of increased sensitivity results from the thermal electrons or charges on the target. An imaging tube with said target inherently provides a narrow dynamic range, i.e., up to 10.sup.3.
Another photographic or television camera also has conventionally been used to detect the image of corpuscular rays formed in accordance with the two-dimensional distribution of incident corpuscles. The former uses photographic films, and the latter uses images tubes to pick up fluorescence of a phosphor layer excited by corpuscular rays.
For a sufficient amount of corpuscular rays, said photographic films or imaging tubes can detect images of the corpuscular rays. For an insufficient amount of corpuscular rays, images resulting from measurement have lower S/N ratios and only data of insufficient quality can be obtained.
If corpuscular rays are incident on an imaging tube to pick up fluorescence of a phosphor excited by corpuscular rays for a long period of time, thermal electrons may be stored in the target of the imaging tube or the contrast of an image taken by the imaging tube is lowered by leakage of charges forming the image. This type of imaging tube provides a narrow dynamic range, i.e., up to 10.sup.3.
The inventors of the present invention present signals identifying the location where electrons are multiplied in single-photoelectron units when these electrons are generated from the photoelectric layer on which photons are incident, and also measure an image with a contrast under lower illumination than that under which any image can be picked up by the conventional television camera, or under low-intensity corpuscular rays per unit time. The inventors have found that two-dimensional images of extremely low intensity can be measured by counting the frequency of occurrence of photoelectrons from the respective locations. There were two problems to be solved for realizing said measuring concept utilizing an imaging tube consisting of a photoelectric layer, micro-channel-plate, and semiconductor position detector.
The first problem lies in dissecting a signal, caused by a unit of photons falling at a specific location with respect to the semiconductor position detector after multiplication by the micro-channel-plate, from thermal noises caused by both the micro-channel-plate and semiconductor position detector themselves.
The second problem lies in that, within certain time spans between generation of succeeding photoelectrons from anywhere on the entire surface of the photoelectric layer in said imaging tube, locations cannot be resolved, although the average of the locations designated within those time spans can satisfactorily be resolved.
Since input signals caused by two or more separate photoelectrons cannot be resolved if the input signals occur within a certain finite period of time, information regarding an assumed location other than the locations where the respective input signals were applied, or information regarding averaged values of these locations, will be issued. That is, if time spans between the generation of succeeding photoelectrons cannot be resolved using the semiconductor position detector, the semiconductor position detector issues noises regardless of the images to be reproduced although input signals are generated from photoelectrons. The second problem lies in such noises inherent in the detected signals.
The problem of noises inherent in the detected signals is not so essential as said first problem of noises inherent in the imaging devices, and the probability that different time spans cannot be resolved is extremely low and is negligible in some cases.
The inventors of the present invention solved the problem of noises inherent in said measuring device as follows:
The multiplication factor of the micro-channel-plate is selected to be suitable for amplifying photoelectrons caused by a pluarality of separate photons until they become greater than the noise level inherent in said measuring device, and also it is selected to be suitable for minimizing variations in the amplification factor. Noises are then eliminated by a pulse amplitude selector with a threshold established between a signal caused by a unit of photons and noises inherent in said measuring device.
The problem of noises inherent in said input signal can be solved as follows: If the micro-channel-plate output caused by two or more photoelectrons is applied to the semiconductor position detector simultaneously or in time spans that cannot be resolved, the output of the semiconductor position detector is greater than that for a single photoelectron. Hence, a pulse amplitude selector with a threshold established between an input signal caused by a unit of photons and noises inherent in said measuring device can be used to separate the desired signal from noises.
An objective of the present invention is to provide measuring devices, for two-dimensional photon-caused picture signals, which can measure with some degree of contrast the two-dimensional image signals that can be separated within the established time spans when a plurality of separate photons are incident on the target surface, i.e., the two-dimensional photon-caused image signals under illumination in the range of 1/10.sup.3 to 1/10.sup.8 of the limit under which images can be picked up by the conventional television camera.
Another objective of the present invention is to provide measuring devices for two-dimensional photon-caused image signals where the problem of noises inherent in said measuring device and the problem of noises inherent in the input signals can be solved.
A further objective of the present invention is to provide measuring devices for two-dimensional photon-caused image signals, the measuring devices having output devices used to output in real time the results or intermediate results of measurement of said two-dimensional photon-caused image signals.
A further objective of the present invention is to provide measuring devices for corpuscular-ray-caused image signals which may reproduce a two-dimensional image with some degree of contrast in such levels that the input signal, in units of electrons generated by the electron emitting plane on which corpuscular rays from the source of the image to be measured are incident, cannot be separated within a time span, or that the input signals in single electron units cannot be separated within a time span.
A further objective of the present invention is to provide a measuring device for corpuscular-ray-caused image signals where the problem of noises inherent in both said measuring device and the input signals has been solved.
A further objective of the present invention is to provide measuring devices for corpuscular-ray-caused image signals, the measuring devices having output devices to output in real time the results or intermediate results of measurement of said corpuscular-ray-caused image signals.